If you’ve ever taken a photo out the airplane window to brag to the folks back home that you just flew over, say, Greenland or the Grand Canyon or a particularly cool-shaped cloud, you may have noticed that there’s always a pesky pinprick of a hole at the bottom of your shot. Before you panic, yes, that hole is very much supposed to be there. Like every other inch of a commercial airplane, the window’s “bleed hole,” as it’s called, is a highly engineered and functional design detail—and it has come a long way over the past century of aviation design history. Long story short: It’s all about pressure.
When an airplane reaches cruising altitude, there’s a significant difference in pressure between the air outside and the air inside the cabin, which is pressurized to mimic the atmosphere at lower altitudes. The earliest jets had square-edged windows, and their corners took the brunt of pressure-induced stress. In fact, those corners often get blamed for a trio of deadly fuselage breakups of de Havilland Comet jets in the 1950s—though many aviation experts today argue that the windows weren’t the sole cause of the crashes. Nevertheless, jet designers began rounding off their windows into the roughly oval shapes we see today, as a way of dispersing the pressure and stress across the surface.
Later engineers improved upon the designs by constructing windows out of three layers of stretched acrylic, rather than glass: a sturdy outer one that’s designed to withstand pressure shifts as the airplane climbs tens of thousands of feet; a middle one that offers added protection in the case of an emergency; and an innermost layer, called the scratch pane, that acts as a barrier, much like your smartphone’s screen protector. If you look closely, you’ll notice that the bleed, or breather, hole cuts through the central pane.
“Aircraft cabins are pressurized to maintain a comfortable and safe environment for passengers and crew, as the air pressure at cruising altitude is much lower than at ground level,” says Munich-based pilot–turned–content creator “Captain Joe,” who you can find on Instagram and YouTube under the name @flywithcaptainjoe.
Bleed holes, Captain Joe explains, “allow for pressure equalization between the space between the panes of the window and the cabin interior. Without these holes, the pressure difference between the cabin and the space between the panes would lead to stress on the window.”
They come in especially handy during unexpected emergencies and help to keep the plane structurally sound. “In the event of an emergency that requires rapid descent or changes in cabin pressure, the bleed holes play a crucial role in preventing the cabin windows from failing,” Captain Joe explains. “If there were no bleed holes, rapid pressure changes could lead to significant stress on the windows, which might result in cracks or even shattering.”
There’s a much less make-or-break secondary purpose for bleed holes too—one that makes those gorgeous views (and photographs) possible. “These holes also aid in preventing condensation or fog from forming between the panes,” Captain Joe says, “which could obstruct the view for the passenger who paid extra for a window seat.”
